An in-joke, also known as an inside joke or a private joke, is a joke whose humour is understandable only to members of an ingroup, that is, people who are in a particular social group, occupation, or other community of shared interest. It is an esoteric joke that is humorous only to those who are aware of the circumstances behind it.

In-jokes may exist within a small social clique, such as a group of friends, or extend to an entire profession such as the film or professional wrestling industries, or a particular sporting field or chess team. Ethnic or religious groups usually have their own in-jokes.[1]

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In-jokes are cryptic allusions to shared common ground that act as triggers; only those who have shared the common ground provide an appropriate response.[2]
An in-joke works to build community, sometimes at the expense of outsiders. Part of the power of an in-joke is that its audience knows that there are those who do not understand the joke.[3]

An in-joke can also be used as a subtext, where people in the know may find humour in something not explicitly spoken. They may even apologize for doing so to a rookie, directly or indirectly stating that what they were laughing at was an in-joke.[4]